1. Field of the Invention
The present invention relates to techniques of reducing in size of a low-pressure mercury vapor lamp such as a compact self-ballasted fluorescent lamp having an arc tube which forms a curved discharge path, without causing an operational failure and a productivity decrease.
2. Related Art
Compact self-ballasted fluorescent lamps that include an arc tube having a curved discharge path are being actively developed in recent years. Examples of an arc tube having a curved discharge path include a U-type arc tube in which a plurality of U-shaped glass bulbs are connected to form one discharge path, and a spiral-type arc tube in which a straight glass bulb is wound in a double spiral.
Such an arc tube is held by a holder so as to be in a standing condition. In detail, the arc tube is held by the holder by bonding both ends of the arc tube to an underside of the holder, i.e. an opposite side of the holder to the arc tube, using a silicone adhesive or the like. An electronic lighting circuit (hereafter simply referred to as “lighting circuit”) is fixed to the underside of the holder, too. A case is attached to the holder so as to cover this lighting circuit.
To further downsize such compact self-ballasted fluorescent lamps, Japanese Patent Application Publication No. H07-085708 discloses the following construction. A protrusion that protrudes into a space surrounded by the arc tube is formed at a center of a top surface of the holder, and a part of the lighting circuit is housed within this protrusion. In this construction, an opening of the holder from the underside into the protrusion has a diameter enough to insert the part of the lighting circuit from the underside into the protrusion.
There is a growing demand for reduction in size of compact self-ballasted fluorescent lamps, as well as other lighting apparatuses. Accordingly, the holder and the case in compact self-ballasted fluorescent lamps tend to be downsized year after year. Meanwhile, there is a limit in downsizing of the lighting circuit, and so the diameter of the opening from the underside into the protrusion remains unchanged. As a result, the opening occupies a large area of the holder, thereby making it impossible to secure a sufficient area for bonding the ends of the arc tube.
In such a case, if a required amount of silicone adhesive is injected to bond the holder and the ends of the arc tube together, the silicone adhesive flows from the opening into the protrusion and adheres to circuit components housed in the protrusion. This causes problems such as an operational failure and a productivity decrease. To avoid this situation, a sufficient area for bonding the ends of the arc tube needs to be secured. This, however, causes the diameter of the opening to decrease, which makes it impossible to insert the part of the lighting circuit into the protrusion.